(1.) Field of the Invention
The present invention relates to an improved product consisting of an elongated non-metallic sheet having a metal embedded in intimate contact there with over the entire length thereof and to a method and apparatus for producing the same. In particular, the present invention relates to a laminated sheet comprising a pair of heat-shrinkable layers fused together with a heating element, such as an electric conductor wire to which electricity is applied, therebetween.
(2.) Description of the Prior Art
A laminated heat-shrinkable sheet with wire embedded therein is well-known as a means for covering and protecting joints of steel pipes in oil pipelines or cables. The sheet is formed by fusing two heat-shrinkable layers of, for example, polyethylene, placed one on top of the other with a wire of, for example, copper therebetween. The layers have the property of being thermally shrinkable in the lengthwise direction. The wire should not be aligned parallel to the shrinking direction but traversely thereto at approximately a right angle and thus has preferably been arranged in a zigzag manner as shown in FIG. 1, in which the numerals 1 and 2 denote the sheet and the wire, respectively. The finished sheet used as a sealing means has a predetermined length and a pair of electric terminals to be connected to a power source. The terminals are on the two ends of the wire exposed from the sheet at the opposite ends of the sheet. The heat-shrinkable sheet is wound around a steel pipe or cable so as to cover the joint portion entirely. Electricity is then applied through the terminals of the wire embedded in the sheet to heat the wire. The arranged hot wire heats the entire sheet to shrink it so that it is in extremely close contact with the joint portion. This close contact protects the joint from surface corrosion and improves sealing of the pipe at the joint. Such results are ensured for a long period even though they depend on the durability of the material forming the heat-shrinkable sheet.
Preferably, the sheet is formed of polyethylene and the wire is of copper.
However, difficulties arise in the manufacture of a polyethylene cover sheet. For example, when the copper wire is embedded in the sheet in a zigzag fashion as shown in FIG. 1, the peripheries of the longitudinal sides (or the rows of turning ends 2a) of the copper wire 2 do not always run parallel to the longitudinal axis of the sheet. As a result, when an electric current is flowed through the copper wire 2, the portion of the sheet 1 where the copper wire is not distributed properly with respect to the peripheries of the longitudinal sides of the sheet 1 is not heated enough to be shrunk. Therefore, when such defective polyethylene cover sheet is applied to a steel pipe joint, a gap is formed between the cover sheet and the joint, allowing water to permeate the joint therethrough causing degradation of the protective and anticorrosive characteristics of the sheet.